Overcoming classical measurement limits through entanglement in photon number: an introduction M. Genovese, G. Adenier, D. Calonico, I. P. Degiovanni, S. Micalizio, I. Ruo Berchera and P. Traina EPJ Web of Conferences, 95 (2015) 03011 Published online: 29 May 2015 DOI: 10.1051/epjconf/20159503011